Interactions between cement and host rock in geological repositories for radioactive waste will result in the formation of a chemically disturbed zone which may affect repository safety. The chemical evolution at the interface between cement (Ordinary Portland Cement: OPC and Low Alkaline Cement: LAC) and mudstone after 11 years of in situ reactions at the Horonobe Underground Research Laboratory is described. Various analytical techniques were used to identify the key reactions at the cement-rock interface, which included cement dissolution, precipitation of secondary minerals such as calcite and C-(A-)S-H phases, cation exchange in clay minerals, and reduction of rock porosity. The results show that the mudstone contacted by both OPC and LAC was altered to depths of a few millimetres, whereas cement alteration was observed over a wider area in LAC.Calcite formation occurred at both interfaces due to the ingress of carbonate ions in the groundwater. A relatively denser calcite layer formed at the OPC interface, suggesting a more favourable environment for calcite precipitation, as suggested by thermodynamic calculations. Furthermore, C-(A-)S-H phases were prevalent in the mudstone, suggesting complex interactions depending on porewater composition, pH, and mineral stability. The study also highlights the effects of cement-mudstone interactions on radionuclide migration, such as reduction of diffusivity due to reduced porosity and enhancement of sorption or incorporation into secondary minerals in the altered mudstone. Overall, the research provides valuable insights into long-term cement-mudstone interactions and their effects on radionuclide behaviour.

Hexavalent chromium (Cr6+) is a toxic carcinogenic pollutant that might be released by the mining and processing of ultramafic rocks and nickel laterites and which requires permanent removal from the contaminated biosphere. Ultramafic material can also serve as a feedstock for the sequestration of CO2 resulting from the growth of new minerals, raising the intriguing proposition of integrated sequestration of both pollutants, CO2 and chromium, into magnesium carbonates. Such a synergistic process downstream of ore recovery and mineral processing could be an elegant proposition for more sustainable utilisation and management of the Earth's resources. We have therefore carried out an experimental and microanalytical study to investigate potentially suitable carbonate minerals. Uptake of chromium in carbonate phases was determined, followed by identification of the crystalline phases and characterisation of the local structural environment around the incorporated chromium centres. The results suggest that neither nesquehonite nor hydromagnesite have the structural capacity to incorporate Cr6+ or Cr3+ significantly at room temperature. We therefore propose that further research into this technology should focus on laboratory assessments of other phases, such as layered double hyroxides, that have a natural structural capacity to uptake both chromium and CO2.

Carbon dioxide, which is a direct product of the combustion of fossil fuels is the main component that enhances the greenhouse effect on Earth. For years, solutions have been developed to reduce the carbon dioxide present in the atmosphere. One of the methods of CO2 reduction is the mineral carbonation of rocks and anthropogenic materials. As a result of the reaction of silicates with CO2, new carbonate minerals that are stable under surface conditions are formed. Ultramafic rocks are considered one of the best substrates for mineral carbonation with the experiments showing relatively high carbonation efficiency. This is because they contain abundant Mg-rich minerals (olivine, serpentine) that readily react with CO2-rich fluids to form Mg-carbonates and silica. However these rocks contain also high abundances of metallic elements, in particular Ni, Cr, and Co, that may be mobilized during the carbonation experiments. This presentation aims at the characterization of selected ultramafic mine waste, in terms of the metallic element content. The goal of our research is to answer the question of whether it is possible to simultaneously bind both CO2 and metallic elements using mineral carbonation experiments. We have chosen three types of ultramafic rocks for our study, two are from abandoned quarries and the third is a mine waste accompanying magnesite exploitation. Their chemical composition is characteristic of ultramafic rocks with high contents of silica and magnesia (both up to 40 wt %), with minor Fe2O3 and alumina (up to 10 and 3 wt % respectively). Metallic element content reaches values of up to 3,400 mg kg-1 for Cr, 2,500 mg kg-1 for Ni, and 125 mg kg-1 for Co. Two rocks represent partially serpentinized peridotites, whose main minerals are olivine and serpentine. The third rock is serpentinite, composed almost exclusively of serpentine group minerals. The minor phases in all the rocks are chlorite and spinel group minerals, with variable chemical compositions ranging from magnesiochromite through Cr-magnetite to magnetite. Peridotites contain amphiboles as minor components whereas carbonates (dolomite and magnesite) and sulfides (mainly Ni-Fe sulfides) were identified as accessory minerals in all the samples. The main Cr-bearing phases are the spinel group minerals, in the case of Ni these are serpentines and olivines, while Co is mainly concentrated in sulfides. Bulk chemical analyses of magnesite veins, naturally occurring in one of the quarries, revealed up to 250 mg kg-1 of Ni, and up to 3 and 5 mg kg-1 of Cr and Co, respectively. Although this indicates that magnesite has the potential to structurally incorporate Ni, further investigations are required to constrain the incorporation mechanism and the potential for the immobilization of Cr and Co in carbonate minerals.AcknowledgmentThe work is funded by the National Science Centre, research project No. 2021/43/B/ST10/01594.

The construction of a repository for geological disposal of radioactive waste will include the use of cement-based materials. Following closure, groundwater will saturate the repository and the extensive use of cement will result in the development of a highly alkaline porewater, pH > 12.5; this fluid will migrate into and react with the host rock. The chemistry of the fluid will evolve over time, initially high [Na] and [K], evolving to a Ca-rich fluid, and finally returning to the groundwater composition. This evolving chemistry will affect the long-term performance of the repository, altering the physical and chemical properties, including radionuclide behaviour. Understanding these changes forms the basis for predicting the long-term evolution of the repository. This study focused on the determination of the nature and extent of the chemical reaction, as well as the formation and persistence of secondary mineral phases within a mudstone, comparing data from sequential flow experiments with the results of reactive transport modelling. The reaction of the mudstone with the cement leachates resulted in small changes in pH with the precipitation of calcium aluminium silicate hydrate (C-(A-)S-H) phases of varying compositions. As the system evolves, secondary C-(A-)S-H phases re-dissolve and are replaced by secondary carbonates. This general sequence was successfully simulated using reactive transport modelling.

Deep geological disposal is the preferred solution for long-term storage of radioactive waste in many countries. In a deep repository, cementitious materials are widely used in the structure and buffer/backfill of the repository for the stabilisation of the hazardous materials. The cement acts as a physical barrier and also contributes chemically to waste containment by buffering the groundwater to a high pH, limiting the solubility of many radionuclides. This paper describes an experimental and modelling study which evaluates the geochemical interaction between young cement leachate (YCL, pH = 13) and a generic hard rock (in this case Hollington sandstone, representing a ‘hard’ host rock) during permeation with the leachate, as it drives mineralogical changes in the system. One-dimensional reactive transport was modelled using a mixing cell approach within the PHREEQC geochemical code to identify the essential parameters and understand and scale up the effect of variations in these parameters on the observed geochemical processes. This study also focused on the effects of variable porosity, reactive surface area and pore volume on improving the modelling of rock alteration in the system compared to conventional models that assume constant values for these properties. The numerical results showed that the interaction between the injected hyper-alkaline leachate and the sandstone sample results in a series of mineralogical reactions. The main processes were the dissolution of quartz, kaolinite and k-feldspar which was coupled with the precipitation of calcium silicate hydrate gel and tobermorite-14A (C–S–H), prehnite (hydrated silicate), saponite-Mg (smectite clay) and mesolite (Na–Ca zeolite). The simulation showed that the overall porosity of the system increased as primary minerals dissolve and no stable precipitation of the secondary C–S–H /C–A–S–H phases was predicted. The variable porosity scenario provides a better fitting to experimental data and more detailed trends of chemistry change within the column. The time and the number of moles of precipitated secondary phases were also improved which was related to greater exposure surface area of the minerals in the sandstone sample to the YCL. Article Highlights The drop in calcium, aluminium and silicate concentrations is mainly due to the formation of calcium silicate hydrate and zeolite minerals as secondary phases. The simulation showed that the overall porosity of the system increased as primary minerals dissolve and no stable precipitation of the secondary C–S–H /C–A–S–H phases was predicted. The dissolution of primary minerals and the precipitation of secondary C–S–H phases had a minimal effect on the pH values, and this was controlled mainly by the initial fluid chemistry. The variable porosity scenario provides a better fitting to experimental data and more detailed trends of chemistry change within the column.

Several repository concepts have been proposed for the disposal of radioactive wastes, some of which include argillaceous (clay-rich) host rocks and cementitious engineered barriers. The presence of hyperalkaline cement pore-fluid results in the destabilization of primary minerals in argillaceous rocks, leading to alteration at the interface between cement/concrete and repository host rock. This phenomenon has implications for radionuclide transport and safety assessment. Data on cement-mudrock interactions from experimental, analogue, and modelling studies have been reviewed, and remaining areas of uncertainty identified. Although a reasonably good understanding of the key processes has been acquired, there are some areas in which uncertainty remains, in particular: system evolution at temperatures above 25 degrees C; the kinetics of secondary mineral growth; the extent of pore-clogging due to secondary mineral formation; the degree to which predicted porosity reduction could impede contaminant migration; and the effect of host rock alteration on contaminant sorption. A multidisciplinary programme of work is likely to be the most productive to elucidate further the key processes and how they operate over different spatial and temporal scales. The ongoing acquisition of geochemical data for model construction, and the testing of geochemical models against analogue and laboratory data, should reduce some of the uncertainties associated with predicting repository evolution, thereby aiding safety case development.

The construction of a repository for the geological disposal of radioactive waste will utilize cement-based materials. Following closure, resaturation will result in the development of a highly alkaline porewater. The alkaline fluid will migrate and react with host rock, producing a chemically disturbed zone (CDZ) around the repository. To understand how these conditions may evolve, a series of batch and flow experiments were conducted with Horonobe mudstone and fluids representative of the alkaline leachates expected from a cementitious repository. Both ordinary Portland cement (OPC) and low alkali cement (LAC) leachates were examined. The impact of the LAC leachates was more limited than the OPC leachates, with experiments using the LAC leachate showing the least reaction and lowest long-term pH of the different leachate types. The reaction was dominated by primary mineral dissolution, and in the case of OPC leachates, precipitation of secondary calcium-silicate-hydrate (C-S-H) phases. Flow experiments revealed that precipitation of the secondary phases was restricted to close to the initial contact zone of the fluids and mudstone. The experimental results demonstrate that a combination of both batch and flow-through experiments can provide the insights required for the understanding of the key geochemical interactions and the impact of transport.

Gaseous impurities, such as O2, are expected to be present within CO2 captured for storage. This could stimulate microbial activity in a geological CO2 storage site which has the potential to lead to operational issues such as injection well blockages, corrosion and oil souring. A series of experiments were carried out to examine the effect of 10 ppm and 100 ppm O2 in an anoxic (CO2 or N2) atmosphere on microbial communities and microbial gas production in laboratory scale experiments. Experiments inoculated with sulphate reducing bacteria enrichments were compared to uninoculated controls. The results show that H2S production is delayed in a CO2 atmosphere compared to the N2 atmosphere. 100 ppm O2 in CO2 resulted in a spike of H2S production as well as greater bacterial biomass when compared to the 10 ppm O2 in CO2 atmosphere. The inoculated N2 experiments showed similar patterns in H2S production and biomass regardless of O2 concentration. These results suggest that a concentration of O2 lower than 100 ppm in CO2 could reduce the potential for microbial growth and H2S production in CO2 storage sites. CH4 production was observed in some microcosms subsequent to H2S production, highlighting the potential for microbial methanogenesis in the in CCS reservoirs.

Current specifications on carbon dioxide (CO2) storage do not take into account the effect of oxygen (O2) present as an impurity, on storage site microbiology. Some microbiology related impacts related to the CCS process include the potential blockage of injection well, corrosion of pipes, oil souring and oil degradation. To investigate this, microcosm experiments were set up using the O2 concentrations of 0 ppm, 10 ppm, 100 ppm and atmospheric. Artificial groundwater and sandstone microcosms were inoculated with a mixed microbial community, incubated for 29 days and regularly sampled for gases produced and sampled at the end of the experiment to analyse the microbiology. Gas chromatography analysis of these microcosms showed no hydrogen sulphide (H2S) production and a variable amount of CO2 production. Microbial analysis of the microcosms show that the microbial inoculum (including sulphate reducing bacteria) was able to survive/grow better in the microcosms with 10 ppm and below compared to the higher levels of O2. The levels of CO2 for 100 ppm and atmospheric levels of O2 were similar indicating the introduction of 100 ppm of O2 could promote aerobic processes. This experiment has shown that small differences in O2 concentrations affects microbial communities relevant to geological storage sites which could cause operational issues. Further investigation is required to properly assess the effect of small O2 changes on H2S production.

In situ sequestration of CO2 in mantle peridotites has been proposed as a method to alleviate the amount of anthropogenic CO2 in the atmosphere. This study presents the results of eight-month long laboratory fluid-rock experiments on representative mantle rocks from the Oman-United Arab Emirates ophiolite to investigate this process. Small core samples (3 cm long) were reacted in wet supercritical CO2 and CO2-saturated brine at 100 bar and 70°C. The extent of carbonate formation, and hence the degree of carbon sequestration, varied greatly depending on rock type, with serpentinite (lizardite-dominated) exhibiting the highest capacity, manifested by the precipitation of magnesite MgCO3 and ferroan magnesite (Mg,Fe)CO3. The carbonate precipitation occurred predominantly on the surface of the core and subordinately within cross-cutting fractures. The extent of the CO2 reactions appeared to be principally controlled by the chemical and mineralogical composition of the rock, as well as the rock texture, with all these factors influencing the extent and rate of mineral dissolution and release of Mg and Fe for subsequent reaction with the CO2. It was calculated that ≈ 0.7 g of CO2 was captured by reacting ≈ 23 g of serpentinite, determined by the mass of magnesite formed. This equates to ≈ 30 kg CO2 per tonne of host rock, equivalent to ≈ 3% carbonation in half a year. However, recycling of carbonate present in veins within the original rock sample could mean that the overall amount is around 2%. The increased reactivity of serpentinite was associated with preferential dissolution of more reactive types of serpentine minerals and brucite, that were mainly present in the cross-cutting veins. The bulk of the serpentinite rock was little affected. This study, using relatively short term experiments, suggests that serpentinite might be a good host rock for CO2 sequestration, although long term experiments might prove that dunite and harzburgite could be an effective in an engineered system of CCSM. Wet scCO2 proved to be chemically aggressive than CO2-saturated brine and its ingress along fractures and grain boundaries resulted in greater host rock dissolution and subsequent carbonate precipitation.

Serpentine minerals serve as a Mg donor in carbon capture and storage by mineralisation (CCSM). The acid-treatment of nine comprehensively-examined serpentine polymorphs and polytypes, and the subsequent microanalysis of their post-test residues highlighted several aspects of great importance to the choice of the optimal feed material for CCSM. Compelling evidence for the non-uniformity of serpentine mineral performance was revealed, and the following order of increasing Mg extraction efficiency after three hours of acid-leaching was established: Al-bearing polygonal serpentine (<5%)≤Al-bearing lizardite 1T (≈5%)<antigorite (24–29%)<well-ordered lizardite 2H1 (≈65%)≤Al-poor lizardite 1T (≈68%)<chrysotile (≈70%)<poorly-ordered lizardite 2H1 (≈80%)<nanotubular chrysotile (≈85%).It was recognised that the Mg extraction efficiency of the minerals depended greatly on the intrinsic properties of crystal structure, chemistry and rock microtexture. On this basis, antigorite and Al-bearing well-ordered lizardite were rejected as potential feedstock material whereas any chrysotile, non-aluminous, widely spaced lizardite and/or disordered serpentine were recommended.The formation of peripheral siliceous layers, tens of microns thick, was not universal and depended greatly upon the intrinsic microtexture of the leached particles. This study provides the first comprehensive investigation of nine, carefully-selected serpentine minerals, covering most varieties and polytypes, under the same experimental conditions. We focused on material characterization and the identification of the intrinsic properties of the minerals that affect particle's reactivity. It can therefore serve as a generic basis for any acid-based CCSM pre-treatment.

Studies of the potential environmental consequences of carbon capture and storage (CCS) have, to date, focused on the physical and chemical impacts of CO2 within stable geological formations together with associated monitoring systems to assure that no significant leakage occurs. If leakage did occur after formal closure of the injection site, this is likely to be restricted to discrete point sources, such as abandoned wells, resulting in locally high concentrations of CO2 in near-surface ecosystems. Consequently, environmental impacts of localised elevated CO2 on terrestrial and marine ecosystems are areas of active research. However, the CO2 storage site could also impact on the deep subsurface microbial ecosystem and biogeochemical processes. This paper describes short pilot studies (2136h/ 89 days) investigating the changes in physical transport properties that are mediated by microbial activity, within samples of sandstone under experimental conditions simulating deep aquifer and reservoir environments in the North Sea. They showed, for the first time, that P. aeruginosa and indigenous microbial populations can survive exposure to saline fluids saturated with CO2. However, little impact on fluid transport under these conditions in these short experiments was observed. It is possible that the microbes require a period of acclimatisation to the extreme environmental conditions generated by the presence of CO2 before any impacts can be detected. Thus, long-term experiments are needed to clarify the role of microbes on rock transport properties.

Investigations of reservoirs, cap-rocks and traps in the Norwegian-Danish Basin have indicated a large potential for geological storage of CO2 onshore Denmark. The possible reservoir rocks include a variety of sandstones with different mineralogical composition, ranging from the Bunter Sandstone Formation containing K-feldspar, clay minerals, calcite, and dolomite as primary reactive minerals to the Gassum Formation containing albite, clay minerals, siderite, and ferroan dolomite as the most reactive minerals.A laboratory and modelling study was carried out to investigate the geochemical response of five potential reservoir rocks to CO2 storage in order to constrain predictions of mineral-CO2 reactions prior to geological storage. The study includes hydrogeochemical experiments, petrographical and mineralogical analyses, and reactive geochemical modelling. During up to 13 months of exposure to CO2 at reservoir conditions (70 degrees C; 20 MPa), the five different rock samples show little mineral reactivity as compared to similar rock samples exposed to N-2 for the same period of time. However, during the period covered by the experiments, dissolution of carbonates present in the host rock is observed both from petrographical analysis and geochemical analysis using speciation calculations. Thus, for the Bunter Sandstone Formation calcite dissolution is apparently taking place in the laboratory experiments while for the Gassum Formation samples ferroan dolomite and siderite dissolution are the dominant mineral dissolution reactions taking place.As a result of the speciation calculations, a procedure for back calculation of chemical analyses to true experimental conditions is suggested in order to reflect the correct saturation state of the pore water with respect to carbonates, primary silicates and aluminosilicates. It is suggested that performing such back calculations is essential to the understanding of the future evolution of the hydrogeochemistry of aquifers aimed at as CO2 storage reservoirs. (C) 2011 Published by Elsevier Ltd.

This study focused on the reactions between carbon dioxide (CO2), porewater and host rock during geological CO2 storage in deep reservoirs. The aim of this work was to provide a well-constrained laboratory experiment reacting known quantities of minerals with CO2-rich fluids, to simulate situations where CO2 is being injected into lithologies deep underground. The experiment was undertaken using a Ti-column 100 cm long, held within a large pressure vessel. The column was packed with a simplified mineral assemblage. The reactant fluid was equilibrated with CO2 at a temperature of 130 degrees C and a pressure of 300 bar, before being pumped into the column held under the same conditions. Fluid was passed along the column at a constant flow rate for approximately 3.5 months. Fluids collected from the outlet end of the column were analysed to provide data on the fate of the dissolved species. On completion of the experiment, the column was then examined for mineralogical changes. The experimental results can be used as a test case for predictive geochemical computer modelling. Such models will help improve our ability to predict the long-term fate of CO2 stored underground. (C) 2011 Published by Elsevier Ltd.